Mitotic checkpoint genes have become widely studied for their roles in development as well as for their potential role in disease such as cancer. The mitotic checkpoint involves a number of different mechanisms to ensure proper cellular division. For example, the spindle assembly checkpoint modulates the timing of anaphase initiation in response to the improper alignment of chromosomes at the metaphase plate. If defects are detected, a signal is transduced to halt further progression of the cell cycle until correct bipolar attachment to the spindle is achieved. Initially identified in budding yeast, several mammalian spindle checkpoint-associated proteins have recently been identified and partially characterized. These proteins associate with all active human centromeres, including neocentromeres, in the early stages of mitosis prior to the commencement of anaphase. The proteins associated with the checkpoint protein complex (BUB1, BUBR1, BUB3, MAD2), the anaphase-promoting complex (Tsg24, p55CDC), and other proteins associated with mitotic checkpoint control (ERK1, 3F3/2 epitope, hZW10), were found to specifically associate with only the active centromere, suggesting that only active centromeres participate in the spindle checkpoint. Saffery R et al., Hum Genet. 107:376-84 (2000).
Tyrosine threonine kinase (TTK), a protein kinase, phosphorylates serine, threonine, and tyrosine hydroxyamino acids (Mills et al,. Biol. Chem. 267:16000-6 (1992)). The kinases most closely related to TTK include SPK1 serine, threonine, and tyrosine kinase, the Pim1, PBS2, and CDC2 serine/threonine kinases, and the TIK kinase (Mills et al. J. Biol. Chem. 267:16000-6 (1992)). The nucleotide and amino acid sequences of human TTK are provided at, for example, GenBank Accession No. M86699. Expression of TTK is markedly reduced or absent in resting cells and in tissues with a low proliferative index (Hogg et al. Oncogene 9:89-96 (1994)). TTK mRNA is expressed in human testis, thymus, bone marrow, and other tissues that contain a large number of proliferating cells and is not detected in tissues that contain few or no dividing cells. TTK expression was detected in several rapidly proliferating cells lines, including various cancer cell lines. TTK expression was also detected and in samples tissue samples from two patients with malignant ovarian cancer (Mills et al., ibid.; Schmandt et al. J. Immunol. 152:96-105 (1994)). TTK expression is correlated with cell proliferation, and plays a role in cell cycle control (Hogg et al., ibid.). Very low levels of TTK mRNA and protein are present in starved cells. When cells are induced to enter the cell cycle, levels of TTK mRNA, protein, and kinase activity increase at the G1/S phase of the cell cycle and peak in G2/M. TTK mRNA levels, as well as kinase activity, drop sharply in early G1, whereas protein levels are largely maintained. TTK is a human homologue of the S. cerevesiae kinase mps 1 and the S. pombe protein mph1, both of which are involved in cell cycle spindle assembly checkpoint, thus indicating that TTK is a spindle checkpoint gene (see, e.g., Cahill et al. Genomics 58:181-7 (1999).
Although mitotic checkpoint impairment has been detected in human cancers (e.g., such impairment is present in about 40% of human lung cancer cell lines) mutations in the MAD mitotic checkpoint genes and the BUB gene family are infrequent. Haruki N et al., Cancer Lett. 162:201-205 (2001); Mimori K et al., Oncol Rep. 8:39-42 (2001); Cahill et al., ibid.). There is thus a need for identification of mitotic checkpoint genes that have a role in human cancers, as they can serve as informative diagnostic and/or prognostic indicators, and therapeutic targets.